The presnet invention relates to a device for monitoring a radio navigation system of the Doppler VOR type.
A VOR system, i.e. a very high frequency omnidirectional radio beacon, enables any aircraft equipped with an appropriate receiver to be supplied with bearing information relative to this beacon on the ground whose geographical position is known. The increase in air traffic, not only national but also international, and in the number of air ways compels all countries to have a radioelectric cover as sure and reliable as possible. This cover is partly provided by the VOR systems which mark out the air routes.
With the VOR system radiating a signal in the metric band, i.e. 108-118 MHz, the quality of this signal depends very largely on the site on which the VOR is installed. In fact, the omnidirectional radiation reflected by obstacles generates errors such that they may make the system unusable. This is why, in hilly sites, the conventional VOR is replaced by the Doppler VOR, whose principles will be outlined hereafter.
Generally, a VOR system causes the phase difference existing between two sinusoidal signals at a frequency of 30 Hz, which modulate a very high frequency carrier frequency, to correspond to the azimuth angle .theta. of the aircraft with respect to the magnetic North of the site of the ground emitter. For this, the antennae forming the VOR system radiate a very high frequency VHF carrier wave modulated in amplitude on the one hand by a first low frequency sinusoidal wave at a frequency of 30 Hz and, on the other hand, by a second low frequency wave at a frequency 9960 Hz, itself modulated in frequency by a sinusoidal signal at a frequency of 30 Hz.
A conventional VOR system radiates two signals:
a "reference" signal, radiated omnidirectionally, formed by an HF carrier modulated in amplitude by a sub-carrier at 9960 Hz, itself frequency modulated by a sinusoidal 30 Hz signal; PA1 a "variable" signal radiated in a figure-of-eight pattern rotating at 30 revolutions per second and in which the LF envelope of the side-bands has a phase which is characteristic of the azimuth.
These two signals combine in space to give the complete VOR signal.
On the other hand, in a Doppler VOR system, the transmission mode for Reference and Variable information is reversed with respect to the conventional VOR. Thus, the 30 Hz reference signal is transmitted omnidirectionally by amplitude modulation of the VHF wave, and the azimuth information is transmitted by frequency modulation at 30 Hz of the two side-bands at .+-.9960 Hz of the carrier wave. These two side-bands are emitted independently of each other by two separate circuits. The omnidirectional pattern transmitting the reference information is radiated by the central antenna, whereas the other pattern containing the azimuth information is obtained by causing to rotate through a circle having a perimeter equal to the modulation index, in wavelengths at the operating frequency, two diametrically opposite antennae each fed by one of the side-bands at .+-.9960 Hz of the carrier frequency. The rotational frequency of these antennae is 30 Hz. For obvious practical reasons, the rotation of these antennae is simulated by switching several antennae, to the number of N, placed in a circle while applying an appropriate weighting function to the feeding of these antennae so as to simulate the progressive movement of the emission point from one antennae to the other.
The use of the standardized VOR system by the International Civil Aviation Organization (ICAO) requires a control device monitoring the integrity of the signal emitted by the VOR and for that, especially the proper operation of the switching of the N antennae, in the case of a Doppler VOR.
In the conventional VOR system, the antennae radiating the two signals for amplitude modulation of the VHF carrier wave are implanted on the same vertical axis, so that it is possible to place a proximity sensor, at a distance of a few wave lengths .lambda. for example. When this sensor is preferably placed at one of the points of intersection of the two principal lobes of the radiation pattern of the side-bands emitted by the antennae, the signal picked up by the sensor in this direction is representative of that emitted in all directions.
In the Doppler VOR system, the signal coming from a sensor close to the antennae does not faithfully represent the radiation emitted to infinity in this direction because of the spread of the antennae network which is too large. The parallax effect gives to this signal a parasite phase modulation of the .+-.9960 Hz side-bands greater than 200.degree. peak to peak for a sensor situated at a distance equal to 5.lambda., i.e. about 14 meters from the central antenna. It prevents then the envelope detection of an amplitude modulated wave. This sensor must then be pushed back to more than 100.lambda. for this modulation to drop below 10 degrees peak to peak (.lambda. being the wavelength at the operating frequency). Furthermore, there exists no privileged direction in which the information received by the sensor is representative of that received in the other direction, contrary to the conventional VOR. To control the Doppler VOR system, it is then necessary to have several distant sensors disposed in different directions. But it is sometimes difficult, if not impossible, to set them up especially in hilly sites. These conditions worsen when, in order to plot a maintenance error curve, twelve at least of these sensors are required. Furthermore, we saw above that the two .+-.9960 Hz side-bands were emitted independently of each other, so that the effective control of the integrity of the emission of the VOR system requires the complete restitution of each of the bands.